I. Field of the Invention
The present invention relates to the field of wireless communications. More particularly, the present invention relates to a novel and improved system for using switching in the repeater to improve diversity between a repeater and a base station.
II. Description of the Related Art
Wireless communication entails the transmission of electromagnetic waves through free space. In a wireless communications system, a base station receives a wireless signal from a communications device, such as a mobile phone. Some base: stations receive diverse versions of the wireless signal by using two antennas. This technique is referred to as receive diversity. Receive diversity improves system performance because one version of the signal may still be available if the other version of the signal fades.
A wireless repeater is sometimes used between the communications device and the base station. The repeater extends the range of the base station by amplifying and re-transmitting the wireless signal between the communications device and the base station. Some repeaters also use receive diversity by receiving two versions of the wireless signal from the communications device. The repeater sums the two versions into a combined signal and transmits the combined wireless signal to the base station.
The current repeater solution is lacking. The summing of the two versions of the wireless signal in the repeater degrades the Signal-to-Noise Ratio (SNR) by at least three decibels in the low SNR conditions. If the transmitter in the repeater malfunctions, then the wireless signal to the base station may be defective or lost. If the wireless signal to the base station experiences significant fading, then the base station cannot retrieve the user information from the wireless signal. In either case, the functionality of the repeater is lost.
Multipath wireless signals are different versions of the same wireless signal that are generated by reflections from structures and natural formations. Multipath signals can have phase shifts that cause the signals to cancel each other out at certain locations. The loss of a signal due to the phase cancellation of multipath signals is known as fading. Fading is a serious problem in wireless systems because it disrupts user communications.
The loss of diversity in the repeater has an impact on wireless systems that use Code Division Multiple Access (CDMA). One form of CDMA is specified in the IS-95 standard approved by the Telecommunications Industry Association, but the invention is not restricted to the form of CDMA specified in this particular standard. CDMA systems transmit and receive wireless signals within a single frequency band and use codes to separate the individual signals. In contrast, other systems use frequency and time division to separate the individual signals. CDMA systems have demonstrated clear advantages in the areas of capacity, voice quality, privacy, and cell hand-off.
CDMA systems require power control. The SNR represents the power of a signal relative to the surrounding noise. An adequate SNR must be maintained so that the signal can be separated from the noise. Since CDMA signals are not divided by frequency or time for a given link direction, the noise component of the ratio includes all other received CDMA signals. If the power of an individual. CDMA signal is too high, it effectively drowns out all other CDMA signals. Thus, power control is used to maintain an equivalent power level for all user signals received at the base station. The power level of these received CDMA signals should be minimized, but still must be strong enough to maintain the appropriate SNR. CDMA systems dynamically control the transmit power of each communications device to maintain the appropriate power level of the received signals at the base station. Dynamic control is applied through open loop and closed loop control techniques that are known in the industry.
The range of the CDMA system is directly related to the common power level of the received signals because each additional signal adds noise to all of the other signals. The user noise component of the SNR is reduced when the average receive power level is lowered. Techniques that decrease CDMA signal power from the communications device directly increase the range of the CDMA system. Receive diversity is one technique used to minimize the required signal power. Lower signal power also lowers the cost of the user communications devices while increasing operational battery life as well as the range.
Diversity improves the reliability and capacity of a CDMA system. Diversity also reduces the signal power required to overcome fading. Prior CDMA systems have used receive diversity at the repeater and base stations. Unfortunately, much of the diversity improvement is lost in the repeater because the diverse versions of the signal are summed. This loss of diversity increases the signal power requirement of the communications device and decreases the range of the CDMA system.
The performance of a wireless communications system could be improved if the diversity between the repeater and the base station is enhanced. For CDMA systems, the range and capacity of the systems can be increased if the signal power can be reduced through improved diversity.
The present invention is a novel and improved system The invention solves the problems discussed above by adding a switching capability in the wireless repeater that provides additional diversity between the repeater and the base station. The repeater does not sum the diverse versions of the wireless signal received by the repeater, but switches between the diverse versions to generate a composite version of the wireless signal. The repeater transmits the composite version of the wireless signal to the base station.
The improved diversity provided by the composite signal increases the reliability of the system. In addition, the, repeater does not incur the three-decibel SNR loss because it does not sum the diverse versions of the wireless signal. Lower SNR increases the capacity and range of wireless base stations that use CDMA. Reduced signal power lowers the cost of the user communications devices while increasing operational battery life and range.
The invention does not require diverse transmitters or power amplifiers in the repeater. The invention does not require special equipment to transmit and receive wireless signals using different polarizations. This reduces the cost of the system. It also makes the system operational in areas where the base station does not have equipment to receive wireless signals using different polarizations.
The invention includes methods and systems for use in a wireless communications system where a communications device transmits a wireless signal. A repeater system receives a first version of the wireless signal using a first receiver system and receives a second version of the wireless signal using a second receiver system. The repeater system switches between the first version of the wireless signal and the second version of the wireless signal to generate a third version of the wireless signal. The repeater system transmits the third version of the wireless signal. A base station system receives the third version of the wireless signal. In some embodiments of the invention, the wireless signal is a Code Division Multiple Access (CDMA) signal. For CDMA communications over an air interface, the switching should occur several times per chip period, such as 4 MHz.